The statistics on State of birth are of value mainly for the information they provide on the historic movements of the native population from one State to another within the United States from the time of birth to the date of the census. Extreme care should be exercised in the use of the statistics as representing or measuring migration, however, since in this connection they indicate only the net result of migration during the widely differing periods of the life of the persons enumerated.
The census figures on State of birth take account, in the matter of migration, only of those persons who have moved from one State to another and are, on the date of the census, living in States other than those in which they were born. The statistics therefore afford no indication of the amount of migration within a given State from rural to urban communities or from one locality to another; nor do they take any account of intermediate moves between the time of a person's birth and the time of the census.

The statistics thus do not indicate the total number of pers.ons who have moved from the State in which they were born to other States, or to any specific State, during any given period of time. Some of those who had gone from one State to another have since died, others have returned to the State in which they were born, and others have gone to still other States or to places outside the United States.

Although the statistics on residence in 1955 apply to a fixed rather than a variable period of time, their uses and limitations are similar in many respects to those on State of birth. The census statistics on mobility provide information on the number of movers, migrants, etc., and on in-migration and out-migration for a given area. The census statistics, however, do not take into account all the different moves that were made in the five-year period; for example, those by persons who moved into or out of a given area but died during the period or returned to their 1955 place of residence. Regardless of the number of moves made, a person is counted only once as mover. Persons were not asked the number of miles they had moved. The census data, however, provide some indication of the relative distance involved in the moves. On the average, a person who moves within a State moves a shorter distance than one who moves to another State in the same division, and he in turn moves a shorter distance than one who moves to another division or region.

Comparison of the characteristics of interstate migrants with the characteristics of other persons gives some indications of the selectivity of interstate migration. It must be borne in mind, however, that the characteristics relate to the period after the move and that some do not necessarily relate to the period before the move.

Combining the two sets of data, as noted previously, gives a condensed lifetime residential history from which one can determine interstate movers between the time of birth and 1955 and between 1955 and 1960. These combinations enable the user to estimate such categories as return migrants and "progressive" migrants, i.e., those who migrated to a different State between birth and 1955 and to a third State between 1955 and 1960.

The terms "in-migrants" and "out-migrants" have been used with reference to migrants into or out of particular areas. In-migrants to an area are migrants who moved into that area from elsewhere in the United States between 1955 and 1960 and were still living there in 1960. Out-migrants from an area are migrants who were living in the area in 1955 and moved out to some other area in the United States where they were in 1960. In-migrants and out-migrants, for example, for a particular geographic division do not include migrants between its States. Thus the sum of the in-migrants to all States in any division will be greater than the number of in-migrants to that division.

Figures on net migration for three periods are shown in table 4. More detailed figures of this type can be derived from some of the other tables. Differentials according to age, sex, color, and educational attainment can be studied by comparing absolute numbers and rates of interstate migration, in-migration, out-migration, and net migration. These differentials should shed some light on the selectivity of interstate migration. Since migration rates vary so much with age, differentials in terms of such social and economic characteristics as education are most meaningful when examined within specific age groups or after standardization for age. The limitation of the statistics for educational attainment to those at least 25 years old means that most migrants in the 1955-196.0 period had completed their formal education (at least in terms of the highest category shown here) before they moved. For moves between birth and 1955, however, it cannot be determined whether the completion of education antedated the move or not.

Somewhat similar considerations apply to the interpretation of age in 1960. In the case of the 1955-1960 period, it can be assumed that the migrants were roughly 2 Â½ years younger at the time of move, than in 1960, whereas the age at the time of migration cannot be determined if one knows merely that it occurred since birth.

The median is presented in connection with the data on age, years of school completed, and income. It is the value which divides the distribution into two equal parts, one-half the cases falling below this value and one-half the cases exceeding this value.

A plus (+) or minus (-) sign after the median indicates that the median is above or below that number. For example, a median of 16+ for years of school completed indicates that the median fell in the interval "college: 4-years or more."

In accordance with census practice dating back to 1790, each person enumerated in the 1960 Census was counted as an inhabitant of his usual place of residence or usual place of abode, that is, the place where he lives and sleeps most of the time. This place is not necessarily the same as his legal residence, voting residence, or domicile, although, in the vast majority of cases, the use of these different bases of classification would produce identical results.

The questions on place of residence in 1955 were supposed to refer to usual residence also. The respondent was not, however, furnished all the rules that the enumerator was instructed to use in determining the respondent's usual residence in 1960. Hence, in some cases, another type of residence may have been reported.

The term "color" refers to the division of population into two groups, white and nonwhite. The color group designated as "nonwhite" includes Negroes, American Indians, Japanese, Chinese, Filipinos, Koreans, Hawaiians, Asian Indians, Malayans, Eskimos, Aleuts, etc. Persons of Mexican birth or ancestry who are not definitely of Indian or other nonwhite race are classified as white.

This category comprises persons born in the United States, the Commonwealth of Puerto Rico, or a possession of the United States; persons born in a foreign country or at sea who have at least one native American parent; and persons whose place of birth was not reported and whose census report, contained no contradictory information, such as an entry, of a language spoken prior to coming to the United States.

The data on years of school completed were derived from the answers to the two questions: (a) "What is the highest grade (or year) of regular school he has ever attended?" and (b) "Did he finish this grade (or year)?" Enumerators were instructed to obtain the approximate equivalent grade in the American school system for persons whose highest grade of attendance was in. a foreign school system, whose highest level of attendance was in an ungraded, school, whose highest level of schooling was measured by readers, or whose training by a tutor was regarded as qualifying under the "regular" school definition. Persons were to answer "No" to the second question if they were attending school, had completed only part of a grade before they dropped out, or failed to pass the last grade attended.
The number in each category of highest grade of school completed represents the combination of (a) persons who reported that they had attended the indicated grade and finished it, and (b) those who had attended the next higher grade but had not finished it.

The questions on educational attainment applied only to progress in "regular" schools. Regular schooling is that which may advance a person toward an elementary school certificate or high school diploma, or a college, university, or professional degree. Schooling that was not obtained in a regular school and schooling from a tutor or through correspondence courses were counted only if the credits obtained were regarded as transferable to a school in the regular school system. Schooling which is generally regarded as not regular includes that which is given in nursery school, in specialized vocational, trade, or business schools; in on-the-job training; and through correspondence courses.
Elementary school, as defined here, includes grades 1 to 8, and high school includes grades 9 to 12. College includes junior or community colleges, regular 4-year colleges, and graduate or professional schools.